1. Field of the Invention
This invention relates to the lifting of cast concrete products and, more specifically, to novel means and a method for providing a lift insert, such as a coil insert, for imbedment in a cast concrete slab or panel for receiving a lifting device to hoist the slab or panel into a desired position.
2. Description of the Prior Art
As described in U.S. Pat. No. 4,074,499, in recent years a form of building construction referred to as the tilt-up slab method has come into rather wide usage. In tilt-up slab building, large concrete slabs are formed either on the floor of the building or on a level ground surface. The concrete slabs are then lifted to a vertical position and interconnected to form the walls of the building.
The attachment to the slabs which are used for lifting them must be removable after the slabs have been lifted into place to facilitate the provision of a smooth surface on the walls of the building. Generally, the lifting attachments, such as a lifting ring and bolt, are installed and removed at the building site, and therefore they are designed to be easily and rapidly installed and removed in order to save maximum time.
The insert into which the bolt is threaded is normally permanently embedded in the slab or panel. When the lifting attachments are removed from the slab, a hole where the insert is located remains in the slab and must be filled to provide a smooth outer surface. The insert should therefore be small enough to be easily and readily concealed after the slab has been installed, and yet should be able to sustain the forces of lifting heavy slabs or panels. Also, it should not interfere with rapid attaching and detaching of the lifting means.
The art has for many years used bolts of conventional types having relatively coarse threads, including nut headed bolts for use with lifting devices into which a hoist can be attached, and also eye bolts which do not require separate lifting rings. A more recent development in the art is a split bolt, such as disclosed in Grayson's U.S. Pat. No. 3,922,946, which can be rapidly inserted without being threaded into an insert in a slab and can then be tightened in place by only a partial revolution.
Such a split bolt comprises a pair of separate longitudinal segments such as would be formed if a conventional bolt is split along its axis and then a portion removed along the axis from each segment so that when the two segments are placed together, they can be inserted axially into an insert sized for threaded reception of the entire bolt. An axially movable key member or wedge is used to replace the removed portions and thereby to spread the two segments into threaded engagement with the insert, and a partial turn of the bolt will tighten it securely in place. The bolt is removed by withdrawing the key, thereby enabling the two segments to be brought together out of threaded engagement with the insert for axial withdrawal without rotation.
The inserts commonly used with both solid bolts and split bolts comprise a coil of a plurality of turns of heavy wire or rod stock. This coil is welded to supporting leg members or the like for locating the coil in proper position in a poured concrete slab or panel, commonly by setting the foot portions of the leg members on the form on which the slab is poured. Experience has established that there is a practical minimum number of turns in a coil and threads on a coil bolt which must be in engagement to assure adequate lifting strength without causing the coil turns to separate and to release the bolt.
A number of alternative means have been used in the art for space-establishing purposes, including plugs of various kinds which serve to prevent access of fluid concrete both to the interior of the coil and into a space immediately adjacent the inner end of the coil, and which can be burnt out or otherwise removed after the concrete is set. Among patents showing a plug of special characteristics for this purpose are U.S. Pat. Nos. 4,074,499 to Mess, 2,880,608 to Boll et al., 3,216,171 to Jenkins, 3,590,538 to Holt and 3,742,661 to Tye. The plugs of all such patents, however, offer certain disadvantages from the standpoint of both cost and simplicity of use, as now briefly summarized.
While U.S. Pat. No. 4,074,499 comprised an improvement over the prior art, in practice, certain difficulties are encountered in inserting the plug and, whether threaded full length or only at the ends, there is never any certainty that the plug is properly seated or threadably matched with the coil insert. The use of an oversized, elastomeric threaded plug, theoretically, should provide a superior seal and the ability to stretch the plug with a rod for insertion from the bottom would appear to facilitate ease of installation. The complete removability of the plug is also advantageous. However, as aforesaid, in practice there is never any certainty that the plug is properly seated or threadably matched with the coil insert. Moreover, intimate alignment of the provided locator lug with the upper end of the coil is both difficult to accomplish on initial insertion and virtually impossible to adjust once insertion is completed.
Boll et al. discloses the use of a filler plug described as composed of sponge rubber or some easily compressed rubber or elastic material allowing the plug to be easily inserted and to retain itself in place, yet to be easily subsequently removable. Boll et al, however, do not teach how to insert or remove the plug and, apparently, it would have to be crammed into place and subsequently dug out piecemeal. Each of the other three patents show a two-part plug which must be separately threaded or otherwise fitted in place at opposite ends of the coil prior to installation, with the outer plug being removed after the concrete has set and with the inner plug remaining in place for partial destruction by the coil bolt as it is threaded into the insert.